The present invention relates primarily to improved capacitors; in particular, the present invention relates to electrochemical apparatus and especially medical devices, both implantable and external to a patient, comprising capacitors that do not require reformation of the oxide. An apparatus incorporating such improved capacitors exhibits extended battery life and/or smaller size as compared to a prior art apparatus using capacitors that require or benefit from periodic reforming of degraded oxide.
Medical devices, and in particular implantable medical devices, have revolutionized the practice of medicine. One critical area of advancement has been in the area of defibrillators and cardioverters. These devices detect abnormal heart rhythms and apply a therapeutic electrical charge to return the heart to a normal rhythm. In general, the devices rely on a relatively low voltage battery to charge a capacitor. At the appropriate time the capacitor discharges the stored electrical energy as a therapeutic pulse.
Battery life in implantable medical devices is a critical concern. In the event that a battery needs replacement, typically the entire device is replaced with a new device subjecting the patient to the attendant discomfort of this invasive procedure. It would be apparent that any extension of battery life would be highly desirable under all circumstances. It is therefore the ultimate desire to extend battery life as much as possible. In lieu of or in addition to extending battery life, reducing the overall device volume of implantable and other medical devices is also highly desirable.
It has long been realized that one limitation to extending battery life is degradation of the capacitor. It has long been realized that capacitors degrade at rest. This degradation has been considered to be due to deterioration of the oxide layer. As capacitors degrade, the amount of battery power required to fully charge the capacitor increases.
There are a myriad of techniques described in the art for alleviating the problems associated with capacitor degradation. U.S. Pat. App. Publ. 2002/0095186 describes maintaining the capacitor at high voltage for about five minutes before discharging through a non-therapeutic load. This is contrary to battery longevity since this charge/discharge sequence provides no useful therapeutic purpose. U.S. Pat. No. 6,096,062 describes a method for testing the leakage current prior to charging the capacitor to peak voltage. The testing and charging, again, represent non-therapeutic battery use that unnecessarily limits the battery life. U.S. Pat. No. 5,899,923 describes a process to set the capacitor reformation interval based on an estimated charge time calculated from measurable electrical parameters. U.S. Pat. No. 5,861,006 describes a process to set the capacitor reformation interval based on the charge time history of the device. U.S. Pat. No. 6,283,985 describes charging and discharging the capacitor through non-therapeutic channels. U.S. Pat. App. Publ. 2001/0047190 describes charging/discharging techniques.
While these techniques are beneficial, they all utilize battery capacity for maintenance thereby robbing battery capacity that would be more beneficially used for therapy. Reducing, or eliminating, non-therapeutic charging of the capacitors would greatly extend the life of existing batteries. It has long been a desire to provide a capacitor that does not require reformation or a maintenance voltage.
Electrolytic capacitors, particularly tantalum-based capacitors, have been prepared utilizing aqueous solutions of ethylene glycol with ionogens such as acetic acid, phosphoric acid and ammonium acetate. Capacitors of this type are exemplified in U.S. Pat. No. 6,219,222. While these capacitors have historically fulfilled many of the necessary requirements they are deficient in their ability to meet the increasing demands related to stabilized oxide layers. The oxide layer in these ethylene glycol based capacitors degrade therefore making them undesirable for use in implantable medical devices for the reasons set forth previously herein.
It has been realized by the inventors, through diligent research, that the degradation in leakage current is associated with re-hydration of the layer of polyphosphate covering the anodic oxide layer upon standing, not physical degradation of the oxide, as previously thought. Phosphoric acid is relied on to improve chemical stability of the oxide layer.
Even with this advanced understanding the dilemma remained for the artisan since no suitable solution was forthcoming.
These apparently unresolved dilemmas are mitigated by the present invention. The present invention is directed to an anodizing electrolyte which mitigates the deficiencies of the prior art. These novel capacitors also have improved properties above and beyond the more highly efficient energy usage. This novel structure allows for improvements in the art of implantable medical devices with extended battery life.